In saucepan, bring maple syrup to boiling. Reduce heat. Simmer, uncovered for 10 to 12 minutes or until reduced to 1 cup.

In medium bowl, beat eggs with electric mixer on medium to high speed until thick and lemon colored about 5 minutes.

In large bowl, beat butter with electric mixer on medium to high speed for 30 seconds. Add granulated and brown sugars; beat to combine. Beat in reduced syrup and eggs. Fold in walnuts, vanilla, rum, and nutmeg. Pour into prebaked crust.

Bake pie on baking sheet in lower third of oven 35 minutes or until set around edges; cool. Serve with rum raisin or vanilla ice cream if desired.